The invention concerns a semifinished wire for a superconducting wire containing Nb3Sn, comprising                a Cu stabilization cladding tube,        a ring-shaped closed diffusion barrier in the inside of the Cu stabilization cladding tube,        a plurality of PIT elements in the inside of the diffusion barrier, each comprising                    a cladding containing Cu,            a small tube, and            a powder core containing Sn                        
The invention also concerns a method of producing such a semifinished wire.
A semifinished wire and a method of producing a semifinished wire are disclosed in EP 1 983 583 B1.
Nb3Sn superconducting wires are used to carry high electric currents quasi without loss, in particular in superconducting magnet coils for generating strong magnetic fields. Nb3Sn achieves higher current densities than other metallic superconducting materials (such as NbTi) and can be used in stronger magnetic fields, but is a relatively brittle material which cannot be plastically deformed (or only to a minimum extent). In particular, Nb3Sn cannot be subjected to wire drawing.
For this reason, Nb3Sn superconducting wires are generally produced in that a semifinished wire is initially produced which can be easily plastically deformed, contains the chemical elements which are required to form Nb3Sn, and is brought into the desired shape for the desired application, e.g. is wound as a solenoid coil. This is followed by reaction annealing during which the brittle superconducting A15 phase is generated.
Nb3Sn superconducting wires are produced in practice in most cases via the “bronze route” in which Nb rods are arranged in a matrix of a CuSn alloy (“bronze”) in the semifinished wire. Semifinished wires of this type can be easily deformed and drawn. In the “internal tin” route, Nb rods and Sn blocks, often in a Cu matrix, are provided in the semifinished wire. In this connection, large amounts of tin can be provided and a correspondingly large percentage of area of Nb3Sn in the finished wire can be achieved. However, tin is relatively soft in comparison with copper, which aggravates handling of the semifinished wire, in particular during drawing. In the “powder in tube” (PIT) route, powder containing Sn is usually arranged in small Nb tubes. Also in this case, large amounts of Sn can be provided and high current carrying capacities can be achieved.
EP 1 983 583 B1 discloses the production of a semifinished wire for an Nb3Sn superconducting wire by arranging a bundle of hexagonal PIT elements in an outer jacket. The outer jacket comprises an outer cladding tube of Cu, a middle cladding tube of Ta and an inner cladding tube of Cu. The outer jacket is produced by hydrostatic extrusion, wherein extrusion is followed by drilling out a core. Each PIT element comprises a Cu jacket and a powder metallurgical core. The outer jacket with the included PIT elements is deformed by means of drawing and intermediate annealing steps and is finally subjected to reaction annealing. The middle cladding tube has a percentage of 10-25% of the overall multifilament and is used for mechanical reinforcement to increase the yield point. For this reason, the finished Nb3Sn superconducting wire shows an improved resistance to the Lorentz forces that occur in magnet systems, thus providing higher current carrying capacities.
EP 1 983 582 A2 describes a similar semifinished wire, in which hexagonal PIT elements are surrounded by hexagonal reinforcing filaments which each comprise a Ta core clad by Cu. The PIT elements and the reinforcing filaments are arranged in an outer jacket of Cu. The reinforcing filaments again serve as mechanical reinforcement.
In addition to superconducting filaments, superconducting wires also contain a normally conducting portion with good electrical conductivity, typically of high purity Cu. The normally conducting portion with good conductivity provides a current path parallel to the superconducting filaments and, in case the superconductor changes into the normally conducting state, takes over the current that has been flowing in the previously superconducting filaments. This prevents destruction of the superconducting wire.
DE 10 2012 218 222 A1 discloses a semifinished wire for an Nb3Sn superconducting wire on the basis of an “internal tin” route. A plurality of hexagonal elements containing Nb, a central structure containing Sn and a Cu matrix are arranged in a Cu cladding tube with a diffusion barrier of Ta and/or Nb on the inner side. The diffusion barrier prevents diffusion of Sn into the Cu cladding tube, thereby preventing deterioration of the electrical properties of this Cu stabilization. A similar Nb3Sn superconducting wire is also disclosed in EP 2 713 413 A2.
DE 10 2004 035 852 B4 discloses an initial filament structure on the basis of “powder in tube” for a superconducting conductor element with Nb3Sn, wherein a plurality of filaments are arranged in a Cu matrix. Each filament comprises a tube which contains Nb and is filled with powder of Sn or an Sn alloy. Each tube is surrounded by a Ta cladding in order to prevent diffusion of Sn into the Cu matrix. A single-core conductor on the basis of “powder in tube” with a Ta barrier is also known from EP 1 705 721 B1.
U.S. Pat. No. 4,411,712 proposes to clad Nb rods with a CuSn alloy and to arrange them in bundles in a tube of high purity Cu. The tube may be provided with a layer of a material that prevents diffusion of Sn, e.g. Ta, in order to obtain high conductivity of the high purity Cu.
WO 2006/011170 A1 moreover discloses a ring-shaped diffusion barrier around a conductive core that is surrounded by MgB2 filaments for an MgB2 superconducting wire.
It is the underlying purpose of the invention to provide a semifinished wire for a superconducting wire containing Nb3Sn and a method for producing a semifinished wire of this type, by means of which a particularly high current carrying capacity can be achieved, at the same time ensuring high conductivity Cu stabilization.